In changing rooms, on the beach etc. it is easy to see who is freeballing as they get changed, but how can you tell if a guy is freeballing if you do not have an opportunity to see him get dressed or undressed? Sometimes if a guy bends over it is possible to get a glimpse down the back of his trousers and get a pretty good idea, but what if you are just following a guy down the road? If his jeans disappear up his butt-crack does that mean he is freeballing, or just wearing loose boxers?
It is often difficult to tell if a guy is freeballing or wearing boxers, breifs are easier to tell. You can look for the swing if his pants are lose or where his buldge is (down the leg may be freeballer) I find the best way to know for sure is to ask the guy it may lead to some fun exploration.
it takes a trained eye - from the front, look for bounce or swing, and any outline of the rim or head (easier to do in lighter weight fabrics)
from behind, there's always the "past the waistline glance", but also look for pantylines (briefs), leglines (boxerbriefs) or bunching (boxers) ... of course, worn-through holes at the pockets are an instant bullseye